1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, an A/F sensor, an NOx sensor, or an HC sensor, for use in, for example, controlling exhaust gas from an internal combustion engine of an automobile. More particularly, the invention relates to a prismatic multilayered gas sensor element; a prismatic ceramic heater of a substantially rectangular cross section for heating the prismatic multilayered gas sensor element; a prismatic multilayered gas sensor element to be arranged integrally with the prismatic ceramic heater; a method for manufacturing the prismatic ceramic heater and the prismatic multilayered gas sensor element; and a gas sensor including the prismatic ceramic heater and/or the prismatic multilayered gas sensor element. The invention further relates to a method for manufacturing a prismatic ceramic heater of a substantially rectangular cross section for heating a gas sensor and a prismatic gas sensor element including the prismatic ceramic heater. Specifically, the invention is applied to a prismatic ceramic heater of a substantially rectangular cross section configured such that a heating resistor is embedded in a ceramic laminate, or to a prismatic gas sensor element configured such that a prismatic ceramic heater including an embedded heating resistor and an oxygen-ion-conductive solid electrolyte layer are arranged in layers. More particularly, the invention relates to a prismatic ceramic heater and a prismatic multilayered gas sensor element, each of which has a substantially rectangular cross section and includes a protective layer for preventing cracking which could otherwise result from contact with a water droplet; a method for manufacturing the prismatic ceramic heater and the prismatic multilayered gas sensor element; and a gas sensor including the ceramic heater and/or the multilayered gas sensor element.
2. Description of the Related Art
Various sensors (hereinafter may be referred to as “gas sensor elements”), such as oxygen sensors, HC sensors, and NOx sensors, have been used to measure the concentration of a certain gas, such as an oxygen, a hydrocarbon (HC) or nitrogen oxides (NOx), contained in high-temperature exhaust gas emitted from an internal combustion engine of an automobile. Generally, such sensors use zirconia ceramic, which is an oxygen-ion-conductive solid electrolyte, to form an oxygen sensor cell for detecting the partial pressure of oxygen contained in exhaust gas. However, zirconia generally becomes oxygen-ion conductive only at 300° C. or higher. Therefore, in order to quickly activate an oxygen sensor cell and/or an oxygen-pumping cell of a gas sensor element, there has been proposed a prismatic multilayered sensor element including a ceramic heater having an embedded heating resistor. The multilayered gas sensor element assumes a prismatic shape for enabling mass production. Specifically, prismatic elements can be mass-produced by the steps of joining a ceramic-heater-forming green sheet and a sensor-cell-forming zirconia green sheet into a multilayered green sheet, and forming a number of prismatic elements from the multilayered green sheet through cutting or blanking.
Exhaust gas passing through an exhaust pipe of an internal combustion engine contains substances other than gas, such as water droplets and oil droplets. Upon contact with such a substance, particularly with a water droplet, a sensor element may crack or break. Since the sensor element or a ceramic heater is exposed to heat of an engine, contact with a water droplet or the like raises a great temperature difference between a portion in contact with the water droplet and its adjacent portion, thereby inducing thermal shock. Such thermal shock may cause a breakage of the sensor element or the ceramic heater. Conventionally, two methods for solving this problem have been employed. In one of these methods, a protector having a number of fine ventilation holes formed therein is deployed around the sensor element or the ceramic heater in such a manner as not to hinder the response of the sensor element. However, the protector fails to provide protection against a liquid substance that readily passes through the ventilation holes. In the other method, the surface of a sensor element is coated with a porous protective layer as disclosed in Japanese Patent Application Laid-Open (kokai) Nos. H04-13961, H07-120429, and 2001-281210.
However, various environmental tests which the present inventors have conducted for feasibility study have revealed the following: even when the surface of a prismatic sensor element is coated with a porous protective layer by a method disclosed or suggested in the above-mentioned publications, contact of a liquid substance, such as a water droplet or oil, with a longitudinally extending edge portion defined by the upper or lower surface and a side surface of the prismatic sensor element, particularly with part of the edge portion located in the vicinity of a heating resistor, induces cracking at the edge portion, and the crack develops to break the prismatic sensor element. This cracking problem arises because the conventional methods do not pay attention to water-induced-shock resistance of such an edge portion. The findings from the tests indicate that, in order to completely prevent cracking or breakage of a prismatic ceramic heater or multilayered sensor element (particularly, a prismatic multilayered sensor element configured such that a ceramic heater and a sensor cell are integrally formed through simultaneous firing), water-induced-shock resistance of a porous protective layer itself must be improved; and the porous protective layer formed on the edge portion is apt to detach or exfoliate upon occurrence of an abrupt thermal change induced by contact with a water droplet or the like.